The problem which led to the present invention involves the movement of workpieces in connection with a forging operation. As is common, workpieces are heated to an elevated temperature prior to forging, most commonly by being placed in a furnace. Once the article is heated to an elevated temperature, it must be rapidly removed from the furnace and placed within the forging apparatus, so it can be worked in the desired manner. It is important that the article be moved quickly and accurately to the forging position. It has been found that this job is particularly suited to a robot manipulator, not least of all because it can be a hot and unpleasant job.
While robots can be regularly programmed to accurately perform such functions there is a problem if the workpiece is not in the position at which the robot is told by its program that it is supposed to be. Furthermore, with relatively small workpieces it is critical that the workpieces be accurately held, so they are accurately forged. Because of the high temperatures involved, certain problems can arise in handling small forging preforms. These include changes in the absolute position of the working surface of the furnace due to distortion with change in temperature or wear, and distortion of the robot arm due to the effects of heat.
Also, when forgings are made of titanium and other reactive metals, the workpiece is often coated with glass and other material to protect it from oxidation. Therefore, while it may seem appropriate to have the robot arm detect the workpiece position by contacting or sensing the workpiece itself, it can be difficult to make this practical. Various optical and electrical sensors which might be useful in other circumstances are not particularly suited for the harsh environment which comprises the interior of a furnace, where temperatures can be in the 1100.degree. C. range or higher.
Also, the glass coating can be adversely affected in its protective function at the location where the robot grips the workpiece. Given this, it is important that the robot accurately and consistently grip a workpiece so that any degraded area will be confined to a non-essential part of the workpiece, typically a portion that is discarded further in the manufacturing process.
There have, of course, been innumerable variations in robot manipulator arms and associated sensors described in the patent and technical literature. But, there are two patents of particular interest to the present invention. The first, USSR Patent Ser. No. 617,257, shows a robot hand clamp for use in manipulating hot workpieces in forging operations. The arm has an electrical sensor which, as indicated above, is prone to degradation in a furnace environment and is not particularly useful when the workpiece is coated with a nonconductive glass coating of uncertain thickness. U.S. Pat. No. 4,129,220 to Peterson et al. shows a bar stock feeding apparatus useful with a metal working machine. As is revealed in the further description herein, the sensor of the present invention is fluidic. The Peterson patent describes a pneumatic feeding system which includes pneumatic sensors and generally reflects pneumatic sensing and gripping devices. However, the Peterson apparatus is not suited for use at high temperature, owing to the details of its construction which include numerous elastomer seals and close fits.
Therefore, there is a need for a simple and reliable manipulator which will accurately grip workpieces which are positioned in a hot environment on a surface of imprecise location.